Function generate_draw_commands 

pub fn generate_draw_commands(
    cmd_buf: &mut Vec<DrawCommand>,
    layout: &LayoutNode,
    info: &InfoNode,
    viewport: (f32, f32),
)

LayoutNode + InfoNode → DrawCommand

viewport is the visible (window) size of the page area; it establishes the root scrollport that page-level position: sticky boxes stick to.